In the wet process for producing phosphoric acid, phosphate rock is digested with concentrated sulfuric acid. The resulting phosphoric acid contains metal impurities which it may be desired to remove or reduce before the phosphoric acid is converted into fertilizers. Magnesium is a particularly troubling contaminant. High concentrations of magnesium in phosphoric acid prohibit the formulation of various grades of fertilizer which are necessary for agricultural crops.
The magnesium problem is becoming acute because a large part of the remaining phosphate rock reserves in the United States, as well as and in other countries, contains unacceptably high levels of magnesium. If such phosphate rock is used to prepare phosphoric acid, the acid will contain so much magnesium that it cannot be used for high grade fertilizers. One expedient has been to blend a high magnesium phosphate rock with phosphate rock of lower magnesium content, but this is a limited and temporary answer to the problem.
Systems employing cation exchange resins for removing metal ions have been proposed for treatment of wet process phosphate rock. A particular objective of such systems is the reduction of the content of magnesium in the acid. Desireably, other metal ions are also reduced, especially calcium.
Folded bed exchange systems, which are also called pulsed-bed systems, have been proposed for treatment of wet process phosphoric acid. (See, for example, U.S. Pat. Nos. 4,228,001 and 4,280,904.) A pulsed-bed system removes increments or pulse amounts of resin from a loading column and transfers the increments through a regeneration and recycle apparatus. One system of this kind is described in U.S. Pat. No. 4,385,993.
For removal of calcium and magnesium ions from a cation exchange resin, regeneration procedures are needed, for example, as described in U.S. Pat. Nos. 4,363,880 and 4,493,907. As there described, calcium is first removed from the resin with a recycled 20 to 50% concentration sulfuric acid. Thereafter, the magnesium ion is removed using a higher sulfuric acid concentration. The removed calcium and magnesium ion are separately crystallized as sulfate salts.
Technical feasibility of the pulsed-bed ion exchange system described, in the cited U.S. Pat. Nos. 4,385,993, 4,363,880 and 4,493,907 has been established. A semi-commercial scale plant using this technology was built and demonstrated in Tunisia.
A by-product of the regeneration and crystallization system is a high concentration sulfuric acid which can be used in a phosphoric acid plant to digest phosphate rock. (See "Industrial Scale-Up of a Phosphoric Acid Purification Process", by James E. Van Wyk, dated Nov. 11, 1986, published by The American Institute of Chemical Engineers, Florida USA).
It is sometimes desirable to treat wet process phosphoric acid by cation exchange to reduce metal impurities at locations other than those associated with a phosphoric acid plant. In such an application, it would be undesirable to have by-product sulfuric acid. This could result in a waste disposal problem. A preferred objective in such a situation is to achieve regeneration with a treating solution which can be continuously reused. A recycle system for this purpose is described in U.S. Pat. No. 4,861,490. The resin regeneration solution is saturated ammonium sulfate, the adsorbed magnesium and calcium and other metal ions being exchanged for ammonium ions. The removed metal ions are crystallized as their sulfate salts. Make-up ammonium sulfate is added to the supernatant from the crystallizer before it is recycled, or, alternatively, the ammonium sulfate is to be formed in situ by adding equivalent amounts of ammonia and sulfuric acid.